Did you mapped the ModelAdministrator to your Administrator role in Project security as shown below. If you did this below, you will be able to see all checkboxes
I double checked the User roles. But somehow the synchronize checkbox is not getting enabled. Also tried to add a new Administrator role, but it also didn’t worked.
What is the rule for the field, is this a visibility rule based on a role?
What happens when you remove the conditional rule for the field? This might provide a clue to answer your question.